The present invention relates to focussing solar collectors, and particularly to those including a solar-radiation reflector and a solar-radiation absorber disposed to receive the solar radiations directly and from the reflector. The invention is particularly useful with respect to solar collectors wherein the absorber is a conduit for a fluid, i.e., a liquid or gas, heat-transfer medium to be heated by the solar radiations, and the invention is therefore described below particularly with respect to this application.
A large number of focussing solar collectors of the above type have been developed of many various configurations. However, efforts are continuously being made to increase the efficiency and to reduce the cost of such collectors, since both of these factors greatly influence the economic feasibility of using collectors in many different applications.
An important factor contributing to the efficiency of a solar collector is its "Concentration Ratio" (C.R.), i.e., the ratio of area of intercepted solar radiations to area of the absorber. It will be appreciated that the area of interception of solar radiations will be at a maximum when the radiation impinges the absorber surface exactly at a right angle, i.e., at an 0.degree. angle of incidence to the normal to the collector surface. However, as a result of the Earth's rotation around the sun, the angle between the sun's rays and the normal to the collector will vary, and therefore for each latitude there is a symmetrical and optimal tilt of the collector. The symmetrical placement is fixed for each different latitude, while the optimal tilt depends on the use of the collector, e.g. for heating or cooling. For stationary concentrating collectors used for cooling, the angle will be such as to have a greater C.R. during summer, and those used for heating will have a greater C.R. during winter. For example, a horizontally tilted collector at the equator is considered symmetrically tilted because the time intervals between the days when the sun is in equinoctial path (and in this particular case the zenith as well) are equal.